建物の安全性及び居住性・使用性の評価において建物の振動特性を 精度よく評価することはきわめて重要である。振動特性は主に固有振動数と減衰定数により決まり，多くの場合観測データに頼っている。観測結果によると，減衰定数は固有振動数に比べてバラッキが大きく定量的な識論は難しい特徴がある。本研究では鉄筋コンクリート造の4階中層建物を対象に，長期間観測された常時微動データを用いて固有振動数と減衰定数を検討した。特 に，減衰定数は減衰曲線の包絡線とウェーブレット変換及び対数減衰法 を用いて評価しており，減衰定数そのものの特性及び評価手法の違い による減衰定数の相違を明らかにした。The exact evaluation of vibration characteristics of buildings is generally very difficult and appears to be of important in the safety and habitability/serviceability design of buildings. Vibration characteristics were determined by the natural frequencies and damping ratio, and when compared to an estimation of the natural frequencies, the damping ratio is the most difficult quantity to evaluate. In many cases, vibration characteristics can be examined through the measurement of an ambient vibration, but the measurement is roughly limited within 2 hours. In the present study, characteristics of natural frequencies and damping ratio of 4-story reinforced concrete building were investigated through long-term measurement of the ambient vibration. One-month-long ambient vibration data were used, and time histories of free vibration were obtained using random decrement technique. Using the time histories, damping ratios were evaluated by three methods including the envelop function, continuous wavelet transform, and logarithmic decrement. It was found that although the natural frequencies show little variations on each measurement, the damping ratios show variations with time and the largest damping ratios were obtained from the continuous wavelet transform, while those from the envelop function showed the smallest value.